Metallurgically bonded stainless steel

ABSTRACT

A steel wire having a stainless steel exterior; the steel wire includes a core region that comprises at least 55 wt. % iron which is metallurgically bonded to a stainless steel coating that consists of a stainless steel region and a bonding region. The stainless steel region can have a thickness of about 1 μm to about 250 μm, and a stainless steel composition that is approximately consistent across the thickness of the stainless steel region. The stainless steel composition includes an admixture of iron and about 10 wt. % to about 30 wt. % chromium. The bonding region is positioned between the stainless steel region and the core region, has a thickness that is greater than 1 μm and less than the thickness of the stainless steel region, and has a bonding composition. The bonding composition includes an admixture of iron and chromium, with a chromium concentration proximal to the stainless steel region that is approximately equal to the chromium concentration of the stainless steel region and has a chromium concentration proximal to the core region that has less than about 5 wt. % chromium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This disclosure claims the benefit of priority as a Continuation of U.S.patent application Ser. No. 13/629,699 filed 28 Sep., 2012 which claimsa benefit of priority to U.S. Provisional Patent Applications No.61/581,239 filed 29 Dec., 2011, and No. 61/581,241 filed 29 Dec., 2011,the disclosures of which are incorporated herein in their entirety.

FIELD OF THE INVENTION

This disclosure is directed to a non-stainless steel productmetallurgically bonded to and carrying a stainless steel outer layer.

BACKGROUND

Steel is one of the most ubiquitous structural materials in the world.Unfortunately, steel is prone to oxidation and thereby structural anddecorative failure. Many techniques have been developed to attempt toprovide a protective coating for steel; these include galvanizing,galvannealing, chromizing, cladding, painting and the like.

A good method for protecting steel is providing a stainless steelcomposition on the exterior of the steel product. Chromizing is a commonmethod for the production of chromium-iron alloys, thereby stainlesssteels, on the surface of steels. Chromizing steel involves thermaldeposition-diffusion processes whereby chromium is diffused into thesteel and produces a varying concentration of chromium in the steelsubstrate. Typically, the surface of the substrate has the highestchromium concentration and as the distance into the substrate increasesthe chromium concentration falls off. Often the chromium concentrationfollows a typical diffusion function, that is, the chromiumconcentration decreases exponentially as a function of distance from thesubstrate. Other chromizing products, such as that described in U.S.Pat. No. 3,312,546, include diffusion coatings that have chromiumconcentrations above 20% that decrease linearly as a function ofdistance into the substrate (see FIG. 1). These high chromium-contentcoatings appear to include a foil or layer of chromium containingmaterial carried by the bulk substrate.

The decreasing concentration of chromium as a function of depth into thesubstrate can affect the corrosion resistance of the material. That is,abrasion of the surface continuously produces new layers with lowerchromium concentrations that are well understood to be less corrosionresistant than the initial surface. This undesirable effect, due to thevariable concentration of chromium in the chromized surfaces, has beenovercome by the advent of explosive cladding.

Explosive welding or cladding of stainless steel onto a carbon steelproduces a stainless steel layer with a consistent compositionmetallurgically bonded to a carbon steel substrate. This techniqueovercomes the variable concentrations associated with chromizing but isseverely limited by the thicknesses of the flying layer, the use of highexplosives, and the metallurgical bond that is formed. Two types ofmetallurgical bonds have been observed in explosively welding metals:under high explosive loading the cross-section to be composed of awave-like intermixing of the base and flying layers and under lowerexplosive loadings the cross-section includes an implantation of grainsof the flying layer into the base layer. E.g. see: Explosive welding ofstainless steel—carbon steel coaxial pipes, J. Mat. Sci., 2012, 47-2,685-695; and Microstructure of Austenitic stainless Steel ExplosivelyBonded to low Carbon-Steel, J. Electron Microsc. (Tokyo), 1973, 22-1,13-18.

The prior art fails to teach a material that includes a stainless steellayer with a consistent composition diffusion bonded to a carbon steelsubstrate. Ideally, such a material would include the corrosionresistance associated with the explosively welded stainless steel andthe deep diffusion bonding observed typical of chromizing applications.

SUMMARY

A first embodiment is a metallurgically bonded stainless steel on asteel form that includes a core region that comprises at least 55 wt. %iron and carries a stainless steel coating. The stainless steel coatingconsists of a stainless steel region and a bonding region. The stainlesssteel region having a thickness of about 1 μm to about 250 μm, andhaving a stainless steel composition that is approximately consistentacross the thickness of the stainless steel region. The stainless steelcomposition includes an admixture of iron and chromium, and includes achromium concentration of about 10 wt. % to about 30 wt. %. The bondingregion is positioned between the stainless steel region and the coreregion, has a thickness that is greater than 1 μm and less than thethickness of the stainless steel region, and has a bonding composition.The bonding composition includes an admixture of iron and chromium, andthe bonding composition has a chromium concentration proximal to thestainless steel region that is approximately equal to the chromiumconcentration of the stainless steel region and has a chromiumconcentration proximal to the core region that has less than about 5 wt.% chromium.

A second embodiment is a steel sheet that includes a first stainlesssteel region having a thickness of about 1 μm to about 250 μm; a firstbonding region positioned between the first stainless steel region and acore region, the first bonding region having a thickness that is greaterthan 1 μm and less than the thickness of the first stainless steelregion; the core region having a thickness of about 100 μm to about 4mm, the core region having a core composition that comprises at least 85wt. % iron; a second bonding region positioned between the core regionand a second stainless steel region; and the second stainless steelregion having a thickness of about 1 μm to about 250 μm. The secondbonding region has a thickness that is greater than 1 μm and less thanthe thickness of the second stainless steel region. The first and secondstainless steel regions have stainless steel compositions that areapproximately consistent across the thickness of the respectivestainless steel regions. Individually, the stainless steel compositionsinclude an admixture of iron and chromium, and a chromium concentrationof about 10 wt. % to about 30 wt. %. The first and second bondingregions have bonding compositions that, individually, comprise anadmixture of iron and chromium, having a chromium concentration proximalto the stainless steel region that is approximately equal to thechromium concentration of the stainless steel region and having achromium concentration proximal to the core region that has less thanabout 5 wt. % chromium.

BRIEF DESCRIPTION OF THE FIGURES

For a more complete understanding of the disclosure, reference should bemade to the following detailed description and accompanying drawingfigures wherein:

FIG. 1 is a plot of chromium concentration as a function of depth forchromized steel (prior art);

FIG. 2 is a plot of chromium and iron concentrations as a function ofdepth for a precursor to a 300 series product;

FIG. 3 is a cross section SEM image of the precursor to the 300 seriesproduct;

FIG. 4 is a plot of chromium concentrations as a function of depth for a300 series product, (solid line) the energy-dispersive X-rayspectroscopy (EDX) data as measured, (dashed line) the EDX datanormalized for the concentration of chromium in the core;

FIG. 5 is a cross section SEM image of the 300 series product;

FIG. 6 is a plot of chromium, nickel, and iron concentrations as afunction of depth for a precursor to a 400 series product;

FIG. 7 is a cross section SEM image of the precursor to the 400 seriesproduct;

FIG. 8 is a plot of chromium and nickel concentrations as a function ofdepth for a 400 series product;

FIG. 9 is a cross section SEM image of the 400 series product; and

FIG. 10 is a schematic of one embodiment described herein.

While specific embodiments are illustrated in the figures, with theunderstanding that the disclosure is intended to be illustrative, theseembodiments are not intended to limit the invention described andillustrated herein.

DETAILED DESCRIPTION

The term “admixture” as related to a plurality of metals, preferablytransition metals, means the metals are intermixed in a given region. Anadmixture can be further described as a solid solution, an alloy, ahomogeneous admixture, a heterogeneous admixture, a metallic phase, orone of the preceding further including an intermetallic or insolublestructure, crystal, or crystallite. Importantly, the term “admixture” asused herein expressly excludes intermixed grains or crystals orinter-soluble materials. That is, the herein described admixtures do notinclude distinguishable grains of compositions that can form a solidsolution, a single metallic phase or the like, for example by heatingthe admixture to a temperature where the grains of compositions couldinter-diffuse. Notably, an admixture can include intermetallic speciesas these intermetallic species are not soluble in the “solute” or bulkmetallic phase. Furthermore, this exclusion of intermixed-intersolublematerials does not limit the homogeneity of the sample; a heterogeneousadmixture can include a concentration gradient of at least one of themetals in the admixture but does not include distinguishable grains orcrystals of one phase or composition intermixed with grains, withcrystals, or in a solute having a second phase of composition in whichthe first phase of composition is soluble.

The noun “alloy”, as related to an admixture of metals, means a specificcomposition of metals, preferably transition metals, with a narrowvariation in concentration of the metals throughout the admixture. Oneexample of an alloy is 304 stainless steel that has an iron compositionthat includes about 18-20 wt. % Cr, about 8-10.5 wt. % Ni, and about 2wt. % Mn. As used herein, an alloy that occupies a specific volume doesnot include a concentration gradient. Such a specific volume thatincludes a concentration gradient would include, as an admixture, aplurality or range of alloys.

Herein, the term “concentration gradient” refers to the regular increaseor decrease in the concentration of at least one element in anadmixture. Typically, a concentration gradient is observed in anadmixture where at least one element in the admixture increases ordecreases from a set value to a higher/lower set value. The increase ordecrease can be linear, parabolic, Gaussian, or mixtures thereof.Typically, a concentration gradient is not a step function. A stepfunction variation is better described as a plurality of abuttingadmixtures.

Layers and/or regions of the herein described materials are referred toas being “metallurgically bonded.” That is, the metals, alloys oradmixtures that provide the composition of the layers and/or regions arejoined through a conformance of lattice structures. No intermediatelayers such as adhesives or braze metal are involved. Bonding regionsare the areas in which the metallurgical bonds between two or moremetals, alloys or admixtures display a conformance of latticestructures. The conformance of lattice structures being the gradualchange from the lattice of one metal, alloy or admixture to the latticeof the metallurgically bonded metal, alloy or admixture.

While terms used herein are typical for the steel industry, the hereindisclosed compositions or regions may consist of, or consist essentiallyof, one or more elements. Notably, steel is considered to be carbonsteel, that is a mixture of at least iron and carbon, and generallycontains up to 2% total alloying elements; including carbon, chromium,cobalt, niobium, molybdenum, nickel, titanium, tungsten, vanadium,zirconium or other metals. Thereby, steel or carbon steel does notconsist of, or consist essentially of, one or more elements but can berandom composition of a variety of elements supported in iron. Whencompositions or regions are described as consisting of, or consistingessentially of, one or more elements, the concentration of non-disclosedelements in the composition or region are not detectable byenergy-dispersive X-ray spectroscopy (EDX) (e.g., EDX has a typicalsensitivity down to levels of about 0.5 to 1 atomic percent). When thecomposition or region is described as consisting of one or moreelements, the concentration of the non-disclosed elements in thecomposition or region is not detectable or within the measurable errorof direct elemental analysis, e.g., by ICP.

The herein described material includes a variety of metallurgicallybonded metals, alloys or admixtures. The composition or concentration oftransition metals in the metals, alloys or admixtures is an importantfeature of the herein described materials. Equally important is thevariation of the compositions or concentrations as a function of depthor distance through the material. Accordingly, herein, the compositionor concentrations of the component metals in the described metals,alloys or admixtures is determined by EDX. Furthermore, herein, when acomposition is termed “approximately consistent” over a distance, in alayer, or in a region, the term means that the relative percentage ofmetals in that distance, layer or region is consistent within thestandard error of measurement by EDX. Preferably, the moving averageover the “approximately consistent” distance, layer or region has aslope of about zero when plotted as a function of concentration (y-axis)to distance (x-axis). More preferably, the concentration (or relativepercentage) of the individual elements in the composition vary by lessthan about 5 wt. %, 4 wt. %, 3 wt. %, 2 wt. %, or 1 wt. % over thedistance.

A first embodiment is a steel form having a stainless steel exterior.The steel form includes a core region which carries a stainless steelcoating; that is, the steel form includes the core region, a bondingregion, and a stainless steel region, where the bonding regionmetallurgically bonds the core region to the stainless steel region.Herein, the steel form is defined by layers or regions that include atleast 55 wt. % iron, notably, the steel form can be coated by, forexample, organic or inorganic coatings but these coatings are not,herein, part of the steel form. The core region of the steel formincludes iron, preferably includes at least 55 wt. % iron. Morepreferably, the iron concentration in the core region is greater than 98wt. %, 99 wt. %, or 99.5 wt. %. Even more preferably, the core region isa carbon steel having a carbon concentration of less than about 0.5 wt.%. Still more preferably, the core region is a carbon steel having acarbon concentration of less than about 0.25 wt. %. Even still morepreferably, the core region is substantially free of chromium and/orsubstantially free of nickel.

The stainless steel coating carried by the core region consists of astainless steel region and a bonding region; the bonding region proximalto the core region and the stainless steel region including thestainless steel exterior. The stainless steel region, preferably, has athickness of about 1 μm to about 250 μm, about 5 μm to about 250 μm,about 10 μm to about 250 μm, about 25 μm to about 250 μm, about 50 μm toabout 250 μm, about 10 μm to about 200 μm, or about 10 μm to about 100μm.

The stainless steel region has a stainless steel composition. Herein, astainless steel composition means that the stainless steel regionincludes an admixture of iron and chromium, specifically, the stainlesssteel composition includes a chromium concentration of about 10 wt. % toabout 30 wt. %, for example the chromium concentration can be about 10wt. %, about 12 wt. %, about 14 wt. %, about 16 wt. %, about 18 wt. %,about 20 wt. %, about 22 wt. %, about 24 wt. %, about 26 wt. %, about 28wt. %, or about 30 wt. %. Preferably, the stainless steel composition isapproximately consistent across the thickness of the stainless steelregion.

The stainless steel composition includes an admixture of iron andchromium, and can further include a transition metal selected from thegroup consisting of nickel, molybdenum, titanium, niobium, tantalum,vanadium, tungsten, copper, and a mixture thereof. In one example, thestainless steel composition comprises an admixture of iron, chromium,and nickel, and comprises a nickel concentration of about 5 wt. % toabout 20 wt. %. In this example, the bonding composition (as describedbelow) consists essentially of iron, chromium and nickel.

In one preferable example, the stainless steel composition has achromium concentration of about 16 wt. % to about 25 wt. %, and nickelconcentration of about 6 wt. % to about 14 wt. %. In another preferableexample, the stainless steel composition consists essentially of iron,chromium and nickel.

In one aspect, the stainless steel composition can have a Cr/Ni massratio of about 70/30 (or about 70 wt. % Cr and about 30 wt. % Ni), orabout 69/31, or about 69.2/30.8. In other aspect, the Cr/Ni mass ratiocan be in a range of about 90/10 to about 50/50, or about 80/20 to about60/40, or about 75/25 to about 65/35. In aspects where the deposited Crand Ni layer(s) on an iron substrate is/are substantially free of iron,the substrate and the deposited layer(s) can be heated to and/or held atan annealing temperature until the Cr/Ni mass ratio of the stainlesssteel surface composition is about 35/15.5, about 30/13, about 25/11,about 20/8.9 or about 18/8 (represented as the wt. % of the elements inthe composition, balance Fe).

In another example, the stainless steel composition can have a chromiumconcentration of about 10.5 wt. % to about 18 wt. %. In still anotherpreferable example, the stainless steel composition consists essentiallyof iron and chromium; and the bonding composition consists essentiallyof iron and chromium.

As stated previously, the stainless steel coating includes the stainlesssteel region and the bonding region which is positioned between thestainless steel region and the core region. The bonding region,preferably, has a thickness that is greater than 1 μm and less than thethickness of the stainless steel region. More preferably, the bondingregion has a thickness of about 5 μm to about 200 μm, about 5 μm toabout 100 μm, or about 10 μm to about 50 μm.

The bonding region has a bonding composition, which includes anadmixture of iron and chromium. The bonding composition further includesa chromium concentration proximal to the stainless steel region that isapproximately equal to the chromium concentration of the stainless steelregion and having a chromium concentration proximal to the core regionthat has less than about 5 wt. %, about 4 wt. %, about 3 wt. %, about 2wt. %, about 1 wt. %, or about 0.5 wt. % chromium. That is, the chromiumconcentration falls through the boding region to a concentration that isless than half of the concentration in the stainless steel region,preferably falls to a concentration that is approximately equal to theconcentration of chromium in the core region. The chromium concentrationgradient in the bonding region can include a linear decrease in chromiumconcentration or a sigmoidal decrease in chromium concentration,preferably the sigmoidal decrease.

Another embodiment is a steel sheet that includes a plurality ofregions, including a first stainless steel region, a first bondingregion positioned between the first stainless steel region and a coreregion, the core region, a second bonding region positioned between thecore region and a second stainless steel region, and the secondstainless steel region (e.g., see FIG. 10). In this embodiment, thefirst stainless steel region has a thickness of about 1 μm to about 250μm; the first bonding region has a thickness that is greater than 1 μmand less than the thickness of the first stainless steel region; thecore region having a thickness of about 100 μm to about 4 mm; the secondstainless steel region having a thickness of about 1 μm to about 250 μm;and the second bonding region having a thickness that is greater than 1μm and less than the thickness of the second stainless steel region.

Preferably, the core region has a core composition that comprises atleast 85 wt. % iron. More preferably, the iron concentration in the coreregion is greater than 98 wt. %, 99 wt. %, or 99.5 wt. %. Even morepreferably, the core region is a carbon steel having a carbonconcentration of less than about 0.5 wt. %. Still more preferably, thecore region is a carbon steel having a carbon concentration of less thanabout 0.25 wt. %. Even still more preferably, the core region issubstantially free of chromium.

The first and second stainless steel regions have stainless steelcompositions that are approximately consistent across the thickness ofthe respective stainless steel regions. These stainless steelcompositions, individually, include an admixture of iron and chromiumwith a chromium concentration of about 10 wt. % to about 30 wt. %, forexample the chromium concentration can be about 10 wt. %, about 12 wt.%, about 14 wt. %, about 16 wt. %, about 18 wt. %, about 20 wt. %, about22 wt. %, about 24 wt. %, about 26 wt. %, about 28 wt. %, or about 30wt. %.

The first and second bonding regions having bonding compositions thatinclude an admixture of iron and chromium. Individually, the bondingregions have chromium concentrations proximal to the respectivestainless steel regions that are approximately equal to the chromiumconcentration of the stainless steel region and have chromiumconcentrations proximal to the core region that has less than about 5wt. %, about 4 wt. %, about 3 wt. %, about 2 wt. %, about 1 wt. %, orabout 0.5 wt. % chromium, preferably the chromium concentrationsproximal to the core region are approximately equal to the chromiumconcentration in the core region. That is, the individual bondingregions each have a chromium concentration gradient. The chromiumconcentration gradient in the bonding region can include a lineardecrease in chromium concentration or a sigmoidal decrease in chromiumconcentration, preferably a sigmoidal decrease.

In one example, the first and second stainless steel composition,individually, comprises an admixture of iron, chromium, and nickel, witha nickel concentration of about 5 wt. % to about 20 wt. %. In thisexample the respective first and second bonding compositions alsoinclude nickel. The Cr/Ni mass ratio can be about 70/30 (or about 70 wt.% Cr and about 30 wt. % Ni), or about 69/31, or about 69.2/30.8. Inother aspect, the Cr/Ni mass ratio can be in a range of about 90/10 toabout 50/50, or about 80/20 to about 60/40, or about 75/25 to about65/35. In still another aspect, the Cr/Ni mass ratio of the stainlesssteel composition can be about 35/15.5, about 30/13, about 25/11, about20/8.9 or about 18/8 (represented as the wt. % of the elements in thecomposition, balance Fe).

In another example, the first and second stainless steel composition,individually, comprises an admixture of iron, chromium, and a transitionmetal selected from the group consisting of nickel, molybdenum,titanium, niobium, tantalum, vanadium, tungsten, copper, and a mixturethereof. In this example, the respective bonding compositions would alsoinclude the selected transition metal(s).

Preferably, the steel sheet that includes the above described regionshas a thickness of about 0.1 mm to about 4 mm. The thickness is thelesser of the height, length, or width of the material. For a typicalsheet, the length and width are multiple orders of magnitude greaterthan the height (or thickness). For example the steel sheet can be asteel coil with a width of about 1 meter to about 4 meters and a lengthof greater than 50 meters.

The individual stainless steel regions can have the same or differentthicknesses; preferably the first and second stainless steel regionshave approximately the same thickness (e.g., ±5%). In one example, thefirst stainless steel region has a thickness of about 10 μm to about 100μm. In another example the second stainless steel region has a thicknessof about 10 μm to about 100 μm. The individual bonding regions can havethe same or different thicknesses; preferably the first and secondbonding regions have approximately the same thickness (e.g., ±5%). Inanother example, the first bonding region has a thickness of about 5 μmto about 100 μm. In still another example, the second bonding region hasa thickness about 5 μm to about 100 μm.

Still another embodiment is a steel form that includes a brushedstainless steel surface carried by a stainless steel region. In thisembodiment, the stainless steel region can have a thickness of about 5μm to about 200 μm, has an approximately consistent stainless steelcomposition that includes an admixture of iron and chromium, and canhave a chromium concentration of about 10 wt. % to about 30 wt. %. Thestainless steel region is carried by a bonding region. Preferably, thebonding region has a thickness of about 5 μm to about 200 μm but lessthan the thickness of the stainless steel region. The bonding regionmetallurgically bonds the stainless steel region to a core region. Thecore region has a core composition that includes at least 85 wt. % iron.The bonding region further includes a bonding composition which includesan admixture of iron and chromium, and a bonding region concentrationgradient that decreases from a chromium concentration proximal to thestainless steel region that is approximately equal to the chromiumconcentration of the stainless steel region to a chromium concentrationproximal to the core region that is less than about 1 wt. %.

In addition to the description of the embodiments provided above, theherein described products are preferably free of plastic deformation.Plastic deformation is the elongation or stretching of the grains in ametal or admixture brought about by the distortion of the metal oradmixture. For example, cold rolled steel with display plasticdeformation in the direction of the rolling. Plastic deformation insteel is easily observable and quantifiable through the investigation ofa cross-section of the steel. Herein, the products are preferablysubstantially free of plastic deformation; that is the products includeless than 15%, 10%, or 5% plastic deformation. More preferably, theproducts described herein are essentially free of plastic deformation;that is, the products include less than 1% plastic deformation. Evenmore preferably, the products described herein are free of plasticdeformation; that is, plastic deformation in the described products isnot observable by investigation of a cross section of the product.

The herein described products which include a stainless steel layer orregion carried by a steel or carbon steel substrate or core can bemanufactured by the low temperature deposition of chromium onto astarting substrate that becomes the core region. The process of forminga metal article that has a stainless steel layer on at least one surfaceof the article, generally, includes selecting an desired stainless steelcomposition, selecting a substrate that is preferably macroscopicallychemically homogeneous, depositing onto the substrate at least onedeposition layer, and forming the stainless steel layer and the bondinglayer by diffusing all or some of the deposited layer into thesubstrate. Notably, a first feature of this embodiment is determining astainless steel layer composition, a core composition, and a bondinglayer concentration gradient.

This embodiment further includes providing a first surface upon whichthe low temperature deposition will occur. Preferably, the first surfaceand the deposition substrate have substantially similar compositions andare comprised of the majority element or the majority elements of thebulk composition. The deposition substrate (e.g., iron, aluminum,titanium, copper, and alloys thereof) should be clean of organic layersand/or oxide layer. Cleaning can be accomplished by heating the firstsurface to a reducing temperature in a reducing atmosphere, for example,a reducing atmosphere that consists essentially of H₂, HN_(x), orCO/CO₂, preferably H₂. The reducing temperature can be any temperaturethat provides a sufficient reaction rate between the reducing atmosphereand the layers on the first surface. Preferably, the reducingtemperature is at least 100° C. below the melting temperature of thefirst surface. The reducing temperature can be in a range of about 200°C. to about 1000° C., about 300° C. to about 900° C., or about 500° C.to about 800° C.

Available techniques for the deposition of chromium onto the startingsubstrate include, but are not limited to, physical vapor deposition,chemical vapor deposition, metal-organic chemical vapor deposition,sputtering, ion implantation, electroplating, electroless plating, packcementation, the ONERA process, salt bath processes, chromium-cryoliteprocesses, Alphatising process, or the like.

In one example, the process is a vapor deposition of metals onto thefirst surface of the deposition substrate. This process can includeheating the first surface to a deposition temperature; for example, thedeposition temperature can be at least 400° C. Onto this heated firstsurface is deposited a first deposition layer that includes at least oneof the minority elements of the stainless steel layer composition (e.g.,chromium). The deposition temperature is preferably at least 200° C., atleast 300° C., at least 400° C., or at least 500° C. In a specificexample, the first surface can be heated to a deposition temperature ina range of about 300° C. to about 1000° C., about 400° C. to about 900°C., or about 500° C. to about 800° C. A first deposition gas compositionis then provided to the heated first surface at a gas compositiontemperature in a range of about 50° C. to about 750° C., about 75° C. toabout 550° C., or about 100° C. to about 450° C.

Depositing the deposition layer includes contacting the first surfacewith a first deposition gas at a deposition temperature. The depositiongas can have a composition that includes a carrier gas and one or morevolatile, deposition layer precursors, can consist of one or morevolatile deposition layer precursors, or can consist of one or morevolatile, deposition layer precursors and H₂. Preferably, depositiongaseous composition includes H₂.

In one preferable example, the chromium is deposited in a non-compactlayer upon the starting substrate. In another preferable example, thechromium is deposited as a layer that consists essentially of chromium.FIGS. 2 and 3 show EDX and scanning electron microscopy (SEM) data ofthe as-deposited chromium layer on the carbon steel substrate. FIG. 2shows the approximate weight percentages of the as-deposited chromiumand iron in the carbon steel substrate. FIG. 3 shows an SEM image of thecross section of the chromium deposited on the carbon steel substrate.In still another preferable example, the chromium is deposited as anadmixture of iron and chromium. In yet another preferable example, thechromium is deposited as an admixture of chromium and an elementselected from the group consisting of nickel, molybdenum, titanium,niobium, tantalum, vanadium, tungsten, copper, and a mixture thereof. Instill yet another preferable example, a plurality of layers of chromiumand an element selected from the group consisting of nickel, molybdenum,titanium, niobium, tantalum, vanadium, tungsten, copper, and a mixturethereof are deposited onto the starting substrate. Volatile, depositionlayer precursor(s) can be, individually, selected from metal alkyls,metal alkylamides, metal amines, metal cyclopentadienyls, metalacetylacetonates, metal carbonyls, metal hydrides, and mixtures thereof.Examples of volatile, deposition layer precursors can be found in TheChemistry of Metal CVD, T. T. Kodas and M. J. Hampden-Smith, 2008John-Wiley and Sons and the Handbook of Chemical Vapour Deposition(CVD), Principles, Technology, and Applications, 2nd Edition, Hugh O.Pierson, 1999, Noyes Publications, each incorporated herein by referencein their entirety. Preferably, the volatile, deposition layer precursoris selected from metal carbonyls, metal hydrides, and combinationsthereof. Examples include but are not limited to Cr(CO)₆, Mn₂(CO)₁₀,Fe(CO)₅, Co₂(CO)₈, Ni(CO)₄, Mo(CO)₆, HCo(CO)₄, and HMn(CO)₅. Morepreferably, the volatile deposition layer precursor is a metal carbonyl.FIGS. 6 and 7 show EDX and SEM data of as-deposited nickel and chromiumlayers on the carbon steel substrate. FIG. 6 shows the approximateweight percentages of the as-deposited chromium, as-deposited nickel,and iron in the carbon steel substrate. FIG. 7 shows an SEM image of thecross section of the chromium and nickel carried by the carbon steelsubstrate.

Deposition from a deposition gas can provide a single deposited elementor a plurality of deposited elements. In one example, the deposition gasprovides a single deposited element from a single volatile depositionlayer precursor (e.g., chromium from Cr(CO)₆). In another example, thedeposition gas includes a second volatile deposition layer precursorand, preferably, provides two deposited elements as the deposition layer(e.g., chromium and nickel from Cr(CO)₆ and Ni(CO)₄). In yet anotherexample, the deposition gas includes a plurality of volatile, depositionlayer precursors and, preferably, provides a plurality of depositedelements as the deposition layer.

Following the deposition of a sufficient quantity of elements onto thesubstrate, the deposition of the first deposition layer is thendiscontinued and then the first surface is held at a processingtemperature (e.g., at least 400° C.) for a processing time. Holding thefirst surface at the processing temperature for the processing timeprovides for the diffusion of the minority elements (e.g., chromium)into the bulk composition and the formation of the stainless steel layerand the bonding layer's alloy concentration gradient.

In one example and following the deposition of the chromium onto thestarting substrate, the deposited chromium and any other depositedmetals are heated to a temperature in a range of about 800° C. to about1200° C., or about 1000° C. Heating at or above the annealingtemperature (e.g., about 1000° C.) can include holding the substrate anddeposition layer at that temperature for an annealing time that issufficiently long to yield a homogeneous stainless steel layer (e.g.,greater than 10 h, 20 h, 30 h, or 40 h). FIGS. 4 and 5 show EDX and SEMdata of a 400 series stainless steel carried by a carbon steel core thatwas made by heating the deposited chromium, e.g., as shown in FIGS. 2and 3. FIG. 4 shows the approximate weight percentage of chromium (asmeasured and normalized) as a function of depth. The stainless steelregion is comparable to a stainless steel composition designationselected from the group consisting of 403 SS, 405 SS, 409 SS, 410 SS,414 SS, 416 SS, 420 SS, and 422 SS. The designation of the compositionof the stainless steel layer can be affected by the concentration oftrace elements in the carbon steel substrate (e.g., nickel, carbon,manganese, silicon, phosphorus, sulfur, and nitrogen), by the additionof one or more trace elements to the as deposited chromium, or by theaddition of one or more trace elements by post treatment of theas-deposited chromium (e.g., by solution, deposition, or ionimplantation methods). FIG. 5 shows an SEM cross section of thestainless steel region, bonding region and core regions notably omittingany observable distinction (e.g., interface) between the respectiveregions.

FIGS. 8 and 9 show EDX and SEM data of a 300 series stainless steelcarried by a carbon steel core that was made by heating the depositedchromium, e.g., as shown in FIGS. 6 and 7. FIG. 8 shows the approximateweight percentages of chromium and nickel as a function of depth. Thestainless steel region is comparable to a stainless steel compositiondesignation selected from the group consisting of 301 SS, 302 SS, 303SS, and 304 SS. Other The designation of the composition of thestainless steel layer can be affected by the concentration of traceelements in the carbon steel substrate (e.g., carbon, manganese,silicon, phosphorus, sulfur, and nitrogen), by the addition of one ormore trace elements to the as deposited chromium, or by the addition ofone or more trace elements by post treatment of the as-depositedchromium (e.g., by solution, deposition, or ion implantation methods).Furthermore, the designation of the composition of the stainless steelis affected by the concentrations of the chromium and nickel in thestainless steel layer; these concentrations can be increased ordecreased independently. FIG. 9 shows a SEM cross section of thestainless steel region, bonding region and core regions notably omittingany observable distinction (e.g., interface) between the respectiveregions.

The determination of the thickness and composition of the stainlesssteel region, bonding region, and optionally the core region isdetermined by cross-sectional analysis of a sample of the hereindescribed products. Preferably, the sample is defined by a 1 cm by 1 cmregion of the face of the product. The sample is then cut through thecenter of the 1cm by 1cm region and the face exposed by the cut ispolished on a Buehler EcoMet 250 ginder-polisher. A five step polishingprocess includes 5 minutes at a force of 6 lbs. with a Buehler 180 Gritdisk, 4 minutes at a force of 6 lbs. with a Hercules S disk and a 6 μmpolishing suspension, 3 minutes at a force of 6 lbs. with a Trident 3/6μm disk and a 6 μm polishing suspension, 2 minutes at a force of 6 lbs.with a Trident 3/6 μm disk and a 3 μm polishing suspension, and then 1.5minutes at a force of 6 lbs. with a microcloth disk and a 0.05 μmpolishing suspension. The cut and polished face is then in an instrumentcapable of energy-dispersive X-ray spectroscopy (EDX). The aboveprovided grinding-polishing procedure may cross-contaminate distinctlayers, as expected the contamination can be consistent across thepolished face. Therefore, a baseline measurement of a region that isfree of a first element may display a greater than baselineconcentration of the first element by EDX (see, for example, FIG. 4).The increase in the base line is dependent on the area of the regionspolished and the concentration of the respective elements in thepolished faces.

What is claimed:
 1. A process for providing a stainless steel regionmetallurgically bonded to a carbon steel substrate, the processcomprising: providing a carbon steel substrate; depositing chromium ontothe carbon steel substrate to provide a chromium layer; and then heatingthe chromium layer and the carbon steel substrate to an annealingtemperature in a range of about 800° C. to about 1200° C. for anannealing time that is sufficiently long to form a homogeneous stainlesssteel region and a bonding region, the bonding region positioned betweenthe stainless steel region and the carbon steel substrate; wherein thehomogeneous stainless steel region has a thickness of about 5 μm toabout 250 μm, and has a stainless steel composition, the stainless steelcomposition comprising an admixture of iron and chromium, and thestainless steel composition comprising a chromium concentration of about10 wt. % to about 30 wt. % that varies by less than 5 wt. %.
 2. Theprocess of claim 1, wherein the chromium layer consists essentially ofchromium.
 3. The process of claim 1, wherein the stainless steelcomposition comprises an admixture of iron, chromium, and a transitionmetal selected from the group consisting of nickel, molybdenum,titanium, niobium, tantalum, vanadium, tungsten, copper, and a mixturethereof.
 4. The process of claim 1, wherein the stainless steelcomposition comprises a chromium concentration of about 10.5 wt. % toabout 18 wt. %.
 5. The process of claim 1, wherein the carbon steelsubstrate has a carbon concentration of less than about 0.5 wt. %. 6.The process of claim 5, wherein the carbon steel substrate has a carbonconcentration of less than about 0.25 wt. %.
 7. The process of claim 1,wherein the stainless steel region has a thickness of about 10 μm toabout 100 μm.
 8. The process of claim 1, wherein the bonding region hasa thickness of about 5 μm to about 100 μm; and has a bonding regioncomposition that comprises an admixture of iron and chromium.
 9. Theprocess of claim 8, wherein the bonding region has a thickness of about10 μm to about 50 μm.
 10. The process of claim 1, wherein the bondingregion has a bonding region composition that comprises an admixture ofiron and chromium.
 11. A process for providing a stainless steel regionmetallurgically bonded to a carbon steel substrate, the processcomprising: providing a carbon steel substrate; depositing chromium ontothe carbon steel substrate to provide a chromium layer; then heating thechromium layer and the carbon steel substrate to an annealingtemperature in a range of about 800° C. to about 1200° C.; and holdingat that temperature for a time sufficiently long to yield a ahomogeneous stainless steel region that has a thickness of about 5 μm toabout 250 μm, and has a stainless steel composition that comprises anadmixture of iron and chromium and a chromium concentration of about 10wt. % to about 30 wt. % that varies by less than 5 wt. %, and a bondingregion positioned between the stainless steel region and the carbonsteel substrate that has a thickness that is greater than 5 μm and lessthan the thickness of the stainless steel region, and has a bondingregion composition that comprises an admixture of iron and chromium. 12.A process for providing a stainless steel region metallurgically bondedto a carbon steel substrate, the process comprising: providing a carbonsteel substrate; depositing nickel onto the carbon steel substrate toprovide a nickel layer; then depositing chromium onto the nickel layerto provide a chromium layer that consists essentially of chromium; andthen heating the nickel layer, the chromium layer, and the carbon steelsubstrate to an annealing temperature in a range of about 800° C. toabout 1200° C. for an annealing time that is sufficiently long to form ahomogeneous stainless steel region and a bonding region, the bondingregion positioned between the stainless steel region and the carbonsteel substrate; wherein the homogeneous stainless steel region has athickness of about 5 μm to about 250 μm, and has a stainless steelcomposition, the stainless steel composition comprising an admixture ofiron, nickel, and chromium, and the stainless steel compositioncomprising a chromium concentration of about 10 wt. % to about 30 wt. %that varies by less than 5 wt. %.
 13. The process of claim 12, whereinthe stainless steel composition comprises an admixture of iron,chromium, and nickel; and the stainless steel composition includes anickel concentration of about 5 wt. % to about 20 wt. %.
 14. The processof claim 13, wherein the stainless steel composition comprises achromium concentration of about 16 wt. % to about 25 wt. % and nickelconcentration of about 6 wt. % to about 14 wt. %.
 15. The process ofclaim 12, wherein the stainless steel composition consists essentiallyof iron, chromium and nickel.